Two-wire field-mounted process device

ABSTRACT

A two-wire field-mounted process device with multiple isolated channels includes a channel that can be an input channel or an output channel. The given input or output channel can couple to multiple sensors or actuators, respectively. The process device is wholly powered by the two-wire process control loop. The process device includes a controller adapted to measure one or more characteristics of sensors coupled to an input channel and to control actuators coupled to an output channel. The controller can be further adapted to execute a user generated control algorithm relating process input information with process output commands. The process device also includes a loop communicator that is adapted to communicate over the two-wire loop.

The present application is a continuation of and claims priority of U.S.patent application Ser. No. 10/400,148, filed Mar. 26, 2003, now U.S.Pat. No. 6,711,446, which is a continuation of and claims priority toU.S. patent application Ser. No. 09/570,268, filed May 12, 2000, nowU.S. Pat. No. 6,574,515, the content of which is hereby incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to process devices. More specifically, thepresent invention relates to field-mounted process control andmeasurement devices.

Process devices are used to measure and control industrial processessuch as the refining of petrochemicals, the processing of food, thegeneration of electric power, and a number of other processes. Processmeasurement devices include process variable transmitters, which measurea process variable such as pressure or temperature and communicate themeasured variable to a process controller. Another type of processdevice is an actuator, such as a valve controller or the like.Generally, process control is accomplished using a combination oftransmitters, actuators, and a process controller that communicateacross a process control loop to a controller. Both types of processdevices interact with the physical process through process interfaceelements. Process interface elements are devices which relate electricalsignals to physical process conditions, and include devices such assensors, limit switches, valve controllers, heaters, motor controllers,and a number of other devices.

The process controller is typically a microcomputer located in a controlroom away from the process. The process controller can receive processinformation from one or more process measurement devices and apply asuitable control signal to one or more process control devices toinfluence the process and thereby control it.

In order to couple to the process, transmitters and actuators aregenerally mounted near the process in the field. Such physical proximitycan subject the process devices to an array of environmental challenges.For example, process devices are often subjected to temperatureextremes, vibration, corrosive and/or flammable environments, andelectrical noise. In order to withstand such conditions, process devicesare designed specifically for “field-mounting.” Such field-mounteddevices utilize robust enclosures, which can be designed to beexplosion-proof. Further, field-mounted process devices can also bedesigned with circuitry that is said to be “intrinsically safe”, whichmeans that even under fault conditions, the circuitry will generally notcontain enough electrical energy to generate a spark or a surfacetemperature that can cause an explosion in the presence of an hazardousatmosphere. Further still, electrical isolation techniques are usuallyemployed to reduce the effects of electrical noise. These are just a fewexamples of design considerations, which distinguish field-mountedprocess devices from other devices, which measure sensor characteristicsand provide data indicative of such characteristics.

Aside from the environmental considerations listed above, anotherchallenge for field-mounted devices is that of wiring. Since processdevices are located near the process far from the control room, longwire runs are often required to couple such devices to the control room.These long runs are costly to install and difficult to maintain.

One way to reduce the requisite wiring is by using two-wire processdevices. These devices couple to the control room using a two-wireprocess control loop. Two-wire devices receive power from the processcontrol loop, and communicate over the process control loop in a mannerthat is generally unaffected by the provision of power to the processdevice. Techniques for communicating over two-wires include 4-20 mAsignaling, the Highway Addressable Remote Transducer (HART®) Protocol,FOUNDATION™ Fieldbus, Profibus-PA and others. Although two-wire processcontrol systems provide wiring simplification, such systems provide alimited amount of electrical power to connected devices. For example, adevice that communicates in accordance with 4-20 mA signaling must drawno more than 4 mA otherwise the device's current consumption wouldaffect the process variable. The frugal power budget of two-wire processdevices has traditionally limited the functionality that could beprovided.

Another way the process control industry has reduced field wiring is byproviding transmitters with two sensor inputs. Such transmitters reducethe number of transmitters/sensor and thereby reduce wiring costs aswell as overall system costs. One example of such a transmitter is theModel 3244MV Multivariable Temperature Transmitter, available fromRosemount Inc., of Eden Prairie, Minn.

Although current multivariable transmitters can reduce wiring costs aswell as overall system costs, they have traditionally been limited toapplications involving two sensors. Thus, in applications with sixteensensors, for example, eight multivariable transmitters would still berequired. Further, if different sensor groups are independentlygrounded, there is a possibility that ground loop errors could occur andadversely affect process measurement.

Current methods used to overcome the problem of coupling a large numberof sensors to the control room include coupling the sensors directly tothe control room. For example, if a situation requires a large number oftemperature sensors, consumers generally create “direct run”thermocouple configurations where thermocouple wire spans the distancebetween the measurement “point” and the control room. These direct runconfigurations are generally less expensive than the cost of obtaining anumber of single or dual sensor transmitters, however, a significantwiring effort is required, and process measurement is rendered moresusceptible to electrical noise due to the long runs.

The process control industry has also reduced the effects of long wireruns on process control by providing field-mounted devices that arecapable of performing control functions. Thus, some aspects of processcontrol are transferred into the field, thereby providing quickerresponse time, less reliance upon the main process controller, andgreater flexibility. Further information regarding such controlfunctions in a field-mounted device can be found in U.S. Pat. No.5,825,664 to Warrior et al, entitled FIELD-MOUNTED CONTROL UNIT,assigned to Rosemount Incorporated.

Although multivariable transmitters and process devices implementingcontrol functions have advanced the art of process control, there isstill a need to accommodate applications requiring a relatively largenumber of sensors, as well as applications requiring enhanced control inthe field.

SUMMARY

A two-wire field-mounted process device is provided. In one embodiment,the process device includes multiple isolated channels includes achannel that can be an input channel or an output channel. The giveninput or output channel can couple to multiple sensors or actuators,respectively. The process device is wholly powered by the two-wireprocess control loop. The process device includes a controller adaptedto measure one or more characteristics of sensors coupled to an inputchannel and to control actuators coupled to an output channel. Theprocess device also includes a loop communicator that is adapted tocommunicate over the two-wire loop. In another embodiment, the two-wirefield-mounted process device includes a controller that is adapted toexecute a user generated control algorithm relating process inputinformation with process output commands. The process device of thisembodiment also includes a loop communicator that is adapted tocommunicate over the two-wire loop.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a process control system employing atwo-wire field mounted process device in accordance with an embodimentof the present invention.

FIG. 2 is a system block diagram of the process device shown in FIG. 1.

FIG. 3 is a system block diagram of a method of providing a processvariable with a field-mounted process device in accordance with anembodiment of the present invention.

FIG. 4 is a system block diagram of a method of operating afield-mounted process device in accordance with an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A two-wire field mountable process device 16 shown in FIG. 1 is providedwhich can be adapted to execute sophisticated user generated controlalgorithms, much like those used with traditional programmable logiccontrollers. Embodiments can include input channels, output channels,and any combination of the two. Generally, each channel is isolated fromthe remainder of the process device. Such isolation removes ground looperrors that currently limit multiple input transmitters. Finally, powermanagement is such that embodiments of the present invention are whollypowered by a two-wire process loop 14. These and other features willbecome apparent upon review of the figures and the related descriptionprovided below.

FIG. 1 is a diagrammatic view of process control system 10 whichincludes control room 12, process control loop 14 and process device 16.Process control system can comprise a single process device coupled tocontrol room 12, however system 10 can also include hundreds of processdevices coupled to one or more control rooms over a number of processcontrol loops.

Control room 12 is typically a facility located away from device 16 thatincludes a microcomputer. A user stationed in control room 12 uses themicrocomputer to interact with various process devices through processcontrol loop 14 and thus controls the process(es) from the control room.For clarity, control room 12 is illustrated as a single block. However,in some control system embodiments, control room 12 may in fact coupleprocess control loop 14 to a global computer network, such as theinternet, so that users worldwide could access process device 16 fromtraditional web browser software.

Loop 14 is a two-wire process control loop. A number of two-wire processcommunication protocols exist for communicating on loop 14, and anysuitable protocol can be used. For example, the HART® protocol, theFOUNDATION™ Fieldbus protocol, and the Profibus-PA protocol can be usedwith embodiments of the present invention. Loop 14 provides power toconnected process devices while providing communication between thevarious devices.

Process device 16 includes cover 17 and base 19 which are preferablyconstructed from a suitable plastic material. Base 19 is adapted to matewith an industry standard DIN rail for mounting. As will be described inmore detail, device 16 is adapted to operate solely upon electricalpower received through loop 14, and is adapted for field-mounting. Thus,device 16 is configured to withstand a relatively large temperaturerange (such as −40 to 85 deg. C.), mechanical vibrations, and relativehumidity in excess of 90%. Such environmental resistance is effectedprimarily through the selection of robust components, as will bedescribed later in the specification. Optional enclosure 18 (shown inphantom) provides added durability and can be any known enclosure suchas a National Electrical Manufacturers Association (NEMA) enclosure, oran explosion-proof enclosure. The process device embodiment shown inFIG. 1 has a number of inputs and outputs, and includes suitablecomputing circuitry (shown in FIG. 2) to execute a user generatedcontrol algorithm. The algorithm is comprised of a number of logicstatements relating specific input events to outputs controlled bydevice 16. The user can change the algorithm either by interfacinglocally with device 16, or by communicating with device 16 over controlloop 14. The algorithm can be generated using conventional logicgeneration software such as Relay Ladder Logic and Sequential FunctionCharts (SFC's). In this sense, device 16 can be considered a two-wirefield-mountable programmable logic controller. Although the descriptionwill focus upon the embodiment shown in FIGS. 1 and 2, such descriptionis provided for clarity, since embodiments employing solely inputs, oroutputs are expressly contemplated. Traditionally devices with thecomputational power of device 16 could not be operated upon two-wireprocess control loops due to prohibitive power constraints.

Process device 16 is coupled to sensors 20, 22, 24, 26, 28 and 30 aswell as actuators 32 and 34. Sensors 20, 22 and 24 are thermocouples, ofknown type, which are coupled to various process points to providevoltage signals based upon process variables at the respective processpoints. Resistance Temperature Devices (RTD's) 26, 28 and 30 are alsocoupled to various process points and provide a resistance that is basedupon process temperature at the respective process points. RTD 26 iscoupled to device 16 through a known three-wire connection andillustrates that various wiring configurations can be used withembodiments of the present invention. Actuators 32 and 34 are coupled toprocess device 16 and actuate suitable valves, switches and the likebased upon control signals from device 16. As noted above, device 16 canexecute a user generated control algorithm to relate specific inputconditions to specific output commands. For example, device 16 may sensea process fluid temperature, and cause actuator 32 to engage a heatercoupled to the process fluid in order to maintain the fluid temperatureat a selected level.

FIG. 2 is a system block diagram of device 16 shown in FIG. 1. Device 16includes loop communicator 36, power module 38, controller 40, andchannels 42, 44, 46, 48, and memory 52. Loop communicator 36 is coupledto process control loop 14 and is adapted for bi-directional datacommunication over loop 14. Loop communicator 36 can include a knowncommunication device such as a traditional FOUNDATION™ Fieldbuscommunication controller or the like. Additionally, communicator 36 caninclude suitable isolation circuitry to facilitate compliance with theintrinsic safety specification as set forth in the Factory MutualApproval Standard entitled “Intrinsically Safe Apparatus and AssociatedApparatus for Use in Class I, II, and III, Division 1 Hazardous(Classified) Locations,” Class Number 3610, published October 1988.

Power module 38 is coupled to loop 14 such that power module 38 providespower to all components of device 16 based upon power received from loop14. Although power module 38 has a single arrow 50 indicating that powermodule 38 provides power to all components, it is noted that such powercan be provided at multiple voltages. For example, power module 38preferably includes a switching power supply that provides electricalpower at a plurality of voltages. Thus, some components such as the A/Dconverters and the isolators can receive a higher voltage such as 4.9volts, while low-power components such the controller 40, memory 52 andloop communicator 36 receive a lower voltage such as 3.0 volts.Additionally, power module 38 is preferably programmable to such anextent that at least one of the voltages provided can be varied. Theselectable nature of power module 38 facilitates power management, whichwill be described later in the specification.

Controller 40 is coupled to memory 52 and executes program instructionsstored therein. Memory 52 is preferably low-power memory operating on3.0 volts, such as the model LRS1331, available from Sharp Electronics.Additionally, memory 52 can be “stacked” memory in which both flashmemory and volatile memory are provided on a single memory module. Theuser generated control algorithm, or “program” executed by controller 40can be changed by a user either by coupling to device 16 locally, or byaccessing device 16 through loop 14. In some embodiments the programincludes instructions that relate process event inputs to outputsdetermined by controller 40. In this sense, device 16 functionssimilarly to a programmable logic controller, which is a device thattypically has not been robust enough for field-mounting, nor able tooperate on the low power levels of two-wire field devices. However, byso providing the functions of a programmable logic controller, much moresophisticated process control algorithms can be implemented through auser friendly interface, such as Relay Ladder Logic or the like.

Controller 40 receives power from module 38, and communicates with loopcommunicator 36. Controller 40 preferably includes a low-powermicroprocessor such as the model MMC 2075 microprocessor available fromMotorola Inc. of Schaumburg, Ill. Additionally, controller 40 preferablyhas a selectable internal clock rate such that the clock rate ofcontroller 40, and thus the computing speed and power consumption, canbe selected through suitable commands sent to device 16 over loop 14.Since higher clock speeds will cause controller 40 to draw more power,clock selection of controller 40, and selection of the voltage levelprovided by power module 38 to controller 40 are preferably performed intandem. In this manner the processing speed and power consumption ofdevice 16 are selectable and vary together.

Controller 40 is coupled to the various channels through interface bus54, which is preferably a serial bus designed for high speed datacommunication such as a Synchronous Peripheral Interface (SPI). Channels42, 44, 46 and 48 are coupled to bus 54 through communication isolators56, 58, 60 and 62, respectively, which are preferably knownoptoisolators, but which can be any suitable isolation devices such astransformers or capacitors. In some embodiments, channels 42, 44, 46 and48 provide data in parallel form, and parallel-serial converters 64 areused to translate the data between serial and parallel forms.Preferably, converters 64 are Universal AsynchronousReceiver/Transmitters (UART's).

Channel 42 is coupled to controller 40, and includes sensor terminals1-n, multiplexer (MUX) 66, analog-to-digital (A/D) converter 68,communication isolator 56, and power isolator 70. It is contemplatedthat communication isolator 56 and power isolator 70 can be combined ina single circuit. Channel 42 is specifically adapted to measure aspecific sensor type such as thermocouples, resistance temperaturedevices, strain gauges, pressure sensors, or other sensor type. Eachsensor terminal is adapted to couple a single sensor, such as athermocouple, to multiplexer 66. Multiplexer 66 selectively couples oneof the sensors to A/D converter 68 such that a characteristic of thesensor (voltage for a thermocouple) is measured and communicated tocontroller 40 through isolator 56 and UART 64. Power for channel 42 isreceived from power module 38 through power isolator 70. Power isolator70 is preferably a transformer, but can be any suitable device. Thoseskilled in the art will appreciate that communication isolator 56 andpower isolator 70 cooperate to ensure that channel 42 is electricallyisolated from the rest of device 16.

Channel 44 is similar to channel 42, and like components are numberedsimilarly. Channel 44 can be configured to measure sensors of adifferent type than that of channel 42. For example, in one embodiment,channel 42 is configured to measure the voltage of thermocouples, andchannel 44 is configured to measure the resistance of RTD's. Each sensorterminal in channel 44 is thus configured to couple to an RTD in a two,three, or four-wire (Kelvin) connection. Because channels 42 and 44 areeach electrically isolated from the rest of device 16, coupling a firstindependently grounded sensor to channel 42, and a second independentlygrounded sensor to channel 44 does not result in the generation ofundesirable ground loop errors. Additionally, since each channel can beconfigured for a specific type of sensor, which can be optimized for aspecific application, parameters such as A/D precision and conversionrate can be tailored for the specific sensor type. For example, achannel designed for high-precision may employ an A/D converter ofconfigured to provide a very high accuracy having a relatively slowerconversion time. Conversely, a channel designed for sensors that measurea process variable that can changes quickly can employ a lower precisionhigh speed A/D converter. Essentially, any sensor input can be switchedbetween operation with resistance-type sensors to operation withvoltage-type sensors based upon configuration information received fromcontroller 40. Controller 40 can provide the configuration informationbased upon information received over loop 14, or through a local input(not shown). Additionally, controller 40 can provide configurationinformation to the channels to adjust analog-to-digital sampling ratesfor each channel, or even for each sensor. This is particularlyadvantageous where sensor rates of change are anticipated based uponinformation known about the process.

Channel 46 is similar to channels 42 and 44, however since channel 46 isconfigured to receive digital inputs, it does not include ananalog-to-digital converter. As illustrated, inputs 1-n are coupled tomultiplexer 66 which conveys the signal of a selected input to bus 54through communication isolator 60 and UART 64. In some digital inputembodiments, the input level may be such that the digital inputs couldbe provided directly to UART 64 through isolator 60. Digital inputs aregenerally indicative of logic-type signals such as contact closure inlimit switches as the like. However, digital inputs 1-n can also becoupled to digital outputs of other process devices such that the inputsrepresent logic signals such as alarms or other Boolean type signals.

Channel 48 is similar to channel 46, but essentially operates in reversecompared to channel 46. Thus, serial information sent to channel 48through the UART is converted into parallel form, and conveyed acrosscommunication isolator 62 to set individual actuator outputs. Thus,logic signals are sent to the terminals labeled ACTUATOR 1-n to causeactuators coupled to such terminals (not shown) to engage or disengageas desired. Such actuators can be any suitable device such as valvecontrollers, heaters, motor controllers and any other suitable device.Essentially, any device that is addressable based upon a logic typeoutput is an actuator.

FIG. 3 is a system block diagram of a method of providing a processvariable with a field-mounted process device in accordance with anembodiment of the present invention. The method begins at block 80 wherea field-mountable process device is wholly powered by a two-wire processcontrol loop. At block 82, the process device is coupled to a firstsensor through a first isolated input channel. A sensor signal isacquired through the first isolated input channel, which signal isindicative of a process variable. At block 84, the process device iscoupled to a second sensor through a second isolated input channel inorder to acquire a second sensor signal. Since the first and secondinput channels are isolated, independent grounding of the first andsecond sensors will not cause undesirable ground loop errors. At block86, the process device computes a process variable based upon one orboth of the sensor signals. Moreover, although the method is describedwith respect to two sensors, a number of additional sensors could beused such that the process variable would be a function of any number ofsensor signals. For example, the process device could average the valuesof the sensor, provide their difference, standard deviation, or anyother appropriate function. At block 88, the computed process device isoutput. Such output can be in the form of information sent over theprocess control loop, a local display, or a local output effectedthrough an output channel.

FIG. 4 is a system block diagram of a method of operating afield-mounted process device in accordance with an embodiment of thepresent invention. At block 80, the device is wholly powered by thetwo-wire process control loop. At block 92, the device receives aninput. Such input can be in the form of signals received through inputchannels, such as the multiple isolated input channels described above,in the form of process information received through the two-wire processcontrol loop, in the form of a local input, or any combination of inputsignals and information. At block 94, the device executesuser-programmable logic to relate the input information to one or moreprocess outputs. The user-programmable logic can be simple or complexalgorithms such as ladder logic, SFC's, fuzzy logic, Adaptive Control,or neural networks and the like. At block 96, the device provides theoutput determined via operation of the user-programmable logic. Theoutput can be a local output, either digital or analog, or the outputcan be sent as information over the two-wire process control loop.

Although the present invention has been described with reference toembodiments of two-wire process device having four channels, workersskilled in the art will recognize that changes may be made in form anddetail without departing from the spirit and scope of the invention,which are defined by the appended claims. For example, although variousmodules have been illustrated and described separately, it is expresslycontemplated that some such modules can be physically embodied together,such as on an Application Specific Integrated Circuit. Further, althoughcontroller 40 is described as a single module, its functions can bedistributed upon multiple microprocessors such that one microprocessorcould provide low-level I/O interaction, such as calibration,linearization and the like, while a second microprocessor executes theuser-generated control algorithm. Additionally, although the descriptionhas focussed upon inputs and outputs being provided through thedisclosed channels, it is expressly contemplated that some processinputs or process outputs could be communicated from/to other processdevices through the process control loop.

1. A two-wire field-mountable process device comprising: a power modulecouplable to a two-wire loop and adapted to wholly power the processdevice with power received from the two-wire loop; a loop communicatorcoupled to the power module, and being couplable to the two-wire loop tocommunicate on the two-wire loop in accordance with the Fieldbus orProfibus protocol, wherein the communicator is configured to provide anoutput on the loop; a controller coupled to the loop communicator andpower module; a first channel operably coupled to the controller, thefirst channel configured to couple to at least a first sensor andconfigured to receive a sensor input; a second channel operably coupledto the controller, the second channel configured to couple to a secondsensor and receive a sensor input; and wherein the first and secondchannels are substantially electrically isolated from each other andfrom the two-wire loop.
 2. The process device of claim 1 wherein thefirst sensor is of a first type, the second sensor is of a second type,and wherein the first and second types are different.
 3. The processdevice 2 wherein the first type is selected from the group of sensorsconsisting of thermocouples and RTDs.
 4. The process device of claim 1and further comprising: a first analog-to-digital converter operablycoupled to the first channel and to the controller, the first converterbeing adapted to convert an analog signal from the first channel, intoat least one digital value at a first conversion rate, and to convey thedigital signal to the controller; and a second analog-to-digitalconverter operably coupled to the second channel and to the controller,the second converter being adapted to convert an analog signal from thesecond channel, into at least one digital value at a second conversionrate, and to convey the digital signal to the controller; and whereinthe first and second conversion rates are different.
 5. The processdevice of claim 4 wherein the first conversion rate is related to a typeof sensor to be coupled to the first sensor terminals.
 6. The processdevice of claim 5 wherein the second conversion rate is related to atype of sensor to be coupled to the second sensor terminals.
 7. Theprocess device of claim 1 and further comprising: a multiplexer coupledto the first channel; and an analog-to-digital converter operablycoupled to the first channel via the multiplexer, the analog-to-digitalconverter being operably coupled to the controller, wherein theanalog-to-digital converter is adapted to convert an analog signal fromthe first sensor terminals, into at least one digital value and toconvey the digital signal to the controller.
 8. The process device ofclaim 1 wherein the controller operates in accordance with a program. 9.The process device of claim 8 wherein the program can be changed. 10.The process device of claim 9 wherein the program can be changed byaccessing the device through the two-wire loop.
 11. The process deviceof claim 8 wherein the program relates an input to the output.
 12. Theprocess device of claim 1 wherein the controller computes a processvariable based upon a sensor input.
 13. The process device of claim 1wherein the process device is in accordance with a DIN standard.
 14. Theprocess device of claim 1 wherein the process device complies with anintrinsic safety specification.
 15. The process device of claim 1including third sensor input operably coupled to the controller, thethird sensor input configured to couple to a third sensor and configuredto receive a third sensor input.
 16. The process device of claim 15including a multiplexer configured to selectively couple the sensors tothe controller.
 17. The process device of claim 16 including an analogto digital converter coupled to an output of the multiplexer and to thecontroller configured to provide a digitized signal to the controller.18. The process device of claim 1 wherein the loop communicator includesisolation circuitry.
 19. The process device of claim 1 including a powermodule which is configured to power circuitry of the first and secondchannels.
 20. A two-wire field-mountable process device comprising: apower module couplable to a two-wire loop and adapted to wholly powerthe process device with power received from the two-wire loop; a loopcommunicator coupled to the power module, and being couplable to thetwo-wire loop to communicate on the two-wire loop; a controller; firstsensor terminals configured to couple to at least a first sensor andconfigured to receive a sensor characteristic; second sensor terminalsconfigured to couple to a second sensor and configured to receive asensor characteristic the second sensor terminals operably coupled tothe controller; third sensor terminals configured to couple to a thirdsensor and configured to receive a third sensor characteristic; amultiplexer configured to selectively couple the first sensor terminalsto the controller or the third sensor terminals to the controller; ananalog to digital converter coupled to an output of the multiplexer andto an input of the controller, the analog to digital converterconfigured to provide a digitized signal to the controller; wherein anoutput from the loop communicator on the two-wire process control loopis related to at least one of the first, second or third sensorcharacteristics; and wherein at least the first and second sensorterminals are substantially electrically isolated from each other andfrom the two-wire loop and the second and third sensor terminals aresubstantially electrically isolated from each other and from the twowire loop.
 21. The process device of claim 20 wherein the first sensoris of a first type, the second sensor is of a second type, and whereinthe first and second types are different.
 22. The process device ofclaim 20 wherein the first sensor is selected from the group of sensorsconsisting of thermocouples and RTDs.
 23. The process device of claim 20comprising: a second analog-to-digital converter operably coupled to thesecond sensor terminals and to the controller.
 24. The process device ofclaim 20 wherein the controller operates in accordance with a program.25. The process device of claim 24 wherein the program can be changed.26. The process device of claim 25 wherein the program can be changed byaccessing the device through the two-wire loop.
 27. The process deviceof claim 24 wherein the program relates a sensor characteristic to theoutput.
 28. The process device of claim 24 wherein the controllercomputes a process variable based upon a sensor characteristic.
 29. Theprocess device of claim 20 wherein the process device is in accordancewith a DIN standard.
 30. The process device of claim 20 wherein theprocess device complies with an intrinsic safety specification.
 31. Theprocess device of claim 20 wherein the loop communicator includesisolation circuitry.
 32. The process device of claim 20 including apower module which is configured to power circuitry of the first sensorterminals, circuitry of the second sensor terminals, and circuitry ofthe third sensor terminals.
 33. The process device of claim 20 whereinincluding a first channel which includes the first and third sensorterminals.
 34. The process device of claim 33 including a second channelwhich includes the second sensor terminals.
 35. A two-wirefield-mountable process device comprising: a power module couplable to atwo-wire loop and adapted to wholly power the process device with powerreceived from the two-wire loop; a loop communicator coupled to thepower module, and being couplable to the two-wire loop to communicate onthe two-wire loop; a controller coupled to the loop communicator andpower module; a first channel operably coupled to the controller, thefirst channel configured to couple to at least a first sensor; a secondchannel operably coupled to the controller, the second channelconfigured to couple to a second sensor; wherein the first and secondsensor channels are substantially electrically isolated from each other;and wherein circuitry of the first channel and circuitry of the secondchannel are powered by the power module.
 36. The process device of claim35 including a connection to a third sensor.
 37. The process device ofclaim 36 including a multiplexer configured to selectively couple thefirst sensor to the controller or the third sensor to the controller.38. The process device of claim 37 including an analog to digitalconverter coupled to an output of the multiplexer and to the controllerconfigured to provide a digitized signal to the controller.
 39. Theprocess device of claim 36 wherein the power module is configured topower circuitry of the connection to the sensor.
 40. The process deviceof claim 35 wherein the power module simultaneously powers the circuitryof the first channel and circuitry of the second channel.